fix: check for sentinel versions of llvm/clang in check.sh (#3326)
fix: check for sentinel versions of llvm/clang in check.sh (#3326)

Fixes #3324 by also checking for the sentinel file left in by tools/modules/update.

Previously check.sh only looked for .git or .svn copies of llvm & clang.  This adds a third method by using our custom sentinel file left over after downloading the zip archive & extracting.
kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe,kythe/kythe